Correct optimization from 56bc6966622e; add a test.

If there are two (or more) heads that point to the same .hgtags
node, we can safely skip parsing the file in all but the last head.
(In 56bc6966622e, we were parsing the file in the first head and
skipping all the others.)

mercurial/localrepo.py

@@ -244,13 +244,10 @@ class localrepository(repo.repository):
             # read the tags file from each head, ending with the tip,
             # and add each tag found to the map, with "newer" ones
             # taking precedence
-            heads = self.heads()
-            heads.reverse()
-            seen = {}
-            for node in heads:
-                f = self.filectx('.hgtags', node)
-                if not f or f.filerev() in seen: continue
-                seen[f.filerev()] = 1
+            f = None
+            for rev, node, fnode in self._hgtagsnodes():
+                f = (f and f.filectx(fnode) or
+                     self.filectx('.hgtags', fileid=fnode))
                 count = 0
                 for l in f.data().splitlines():
                     count += 1
@@ -269,6 +266,24 @@ class localrepository(repo.repository):
 
         return self.tagscache
 
+    def _hgtagsnodes(self):
+        heads = self.heads()
+        heads.reverse()
+        last = {}
+        ret = []
+        for node in heads:
+            c = self.changectx(node)
+            rev = c.rev()
+            try:
+                fnode = c.filenode('.hgtags')
+            except repo.LookupError:
+                continue
+            ret.append((rev, node, fnode))
+            if fnode in last:
+                ret[last[fnode]] = None
+            last[fnode] = len(ret) - 1
+        return [item for item in ret if item]
+
     def tagslist(self):
         '''return a list of tags ordered by revision'''
         l = []

tests/test-tags

@@ -60,3 +60,19 @@ hg commit -m "head" -d "1000000 0"
 hg tags
 hg tip
 
+# tags from later heads override previous ones
+cd ..
+hg init t2
+cd t2
+echo foo > foo
+hg add foo
+hg ci -m 'add foo' -d '1000000 0'      # rev 0
+hg tag -d '1000000 0' bar              # rev 1
+echo >> foo
+hg ci -m 'change foo 1' -d '1000000 0' # rev 2
+hg up -C 1
+hg tag -r 1 -d '1000000 0' bar         # rev 3
+hg up -C 1
+echo >> foo
+hg ci -m 'change foo 2' -d '1000000 0' # rev 4
+hg tags

tests/test-tags.out

@@ -36,3 +36,7 @@ user:        test
 date:        Mon Jan 12 13:46:40 1970 +0000
 summary:     head
 
+1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+tip                                4:36195b728445
+bar                                0:b409d9da318e